RouterGod Online Magazine reporter Michael
McClenney was not able to finish the interview with Gillian Anderson. It
was learned that his Ford Explorer has Firestone tires on it. No one can
say what his life expectancy is, only that it is not measured in months.
Michael is getting his affairs in order and we send condolences to his
family. Continuing the interview with Miss Anderson is RouterGod Microsoft
Exchange guru, Irfan Ali. Let's join Irfan as he learns more about LAN
switching from Gillian Anderson:

An impeccable housekeeper, Gillian wears this dress
and drags herself along the floor to pick up dust bunnies
and hairballs from her cat, Punkin.

Is she into chicks? We can only speculate.

Perhaps she's not gay, perhaps she's only into herself.
Again, we can only speculate...

Great! You were talking about vlans earlier and how a router is
required to move packets between vlans. What would you like to
talk about now?

Gillian

I'd like to talk about a serious problem with switches.
Switches by default forward broadcasts. If your switch topology
contains redundant paths or bridge loops, a broadcast storm of Biblical proportions
could result. Routers on the other hand, do not have this problem
because they do not forward broadcasts. Switches use something
called the Spanning Tree Protocol to ensure a loop free topology.
The world of Spanning Tree has two warring factions, each bent on the
destruction of the other. The two factions are the IEEE and
DEC. These two spanning tree protocols are not compatible and
should never be enabled on the same network simultaneously. Use
the IEEE version of Spanning Tree unless you work at DEC.

Irfan

I've heard about the spanning tree protocol. I've never really
thought it was that important.

Gillian

I know you just said that to get me fired up, problems relating to
spanning tree can easily create broadcast storms the lock up routers so
bad you can't even get a console connection! You must understand
spanning tree or STP as it's called if you are going to be a big city
Cisco engineer!

Irfan

I knew I'd get you going! Tell us about STP. Let's hear
the gory details!

Gillian

OK, you asked for it. Right off the bat, a switch is really a
group of bridges that are wired up in a star pattern. Every port
on a switch is a bridge and every behind every port they are wired
together. So you can say switch port or bridge but you are talking
about the same thing. Spanning tree is a bridging protocol and as
such is applied to individual ports on a switch.

Irfan

Do switches use STP to talk to other switches?

Gillian

Don't think "switch", the switch is a box that houses the
bridge ports. It's the bridge ports that use Spanning Tree to talk
to other bridge ports. When Spanning Tree is running on your
switch, what is going on behind the scenes is that every active port on
your switch is sending out little packets called Bridge Protocol Data
Units or BDPUs. Sometimes BDPUs are called "hello
messages". These BDPUs are beaconed out every 2 seconds by
default. When you introduce a port into a spanning tree
environment, it listens for these BDPUs to learn about the spanning tree
topology.

Irfan

How is the topology created by spanning tree?

Gillian

The entire topology must resemble a tree to work properly and avoid
loops. A tree has small branches that flow downward into larger
branches to a common, single root. If you have your switches wired
together that contains multiple paths or that is "meshy",
spanning tree will run what's called the Spanning Tree Algorithm and
choose which ports offer the best path to the root of the tree.
Those ports which spanning tree selects as the way out of the local
network will be put in a forwarding state and the other, non optimal
ports will be put in a blocking state.

Irfan

Is there a central SPT server that does this calculation, then tells
all the ports in the enterprise what state to become?

Gillian

No, as long as each switch is using the same version of spanning
tree, IEEE for example, they all know the rules and can read the BDPUs
and determin which ports are blocked and which ports can forward.

Irfan

What other states are there that a port can be in?

Gillian

Well at first when a port comes up it goes int the Listening
State. It listens for BDPUs and by examing the contents of the
BDPUs and figures out the topology of the existing network. After
the listening state, the port may decide that it should go into a
Blocking State. In the blocking state, no data can pass through
the port but it is still examining BDPUs. The port remains in the
Listening State for however long a duration is specified by the Forward
Delay Timer. The Forward Delay Timer is set to 15 seconds by
default.

Irfan

So it could take a port up to 15 seconds to begine forwarding data?

Gillian

Not so fast, Buckaroo! After the 15 second listening state, if
the port does not go into blocking state, it then spends another
leisurely 15 seconds in the Learning State. In the Learning State
the port is learning MAC addresses and adding those entries into it's
CAM table. After the Learning State is complete, then the port
goes into Forwarding State and can transmit data.

Irfan

30 seconds before the port will send data?!?!? That just won't
do! If I have a single workstation plugged into a port on a switch
and it takes 30 seconds, Windows will not wait that long, my workstation
will never contact the DHCP server in time! My workstation will
not get an IP address! All is lost! We are doomed!

Gillian

Relax, Irfan! There's a work around! When you have a
single workstation or server plugged into a port, you can selectively
disable the spanning tree protocol on that port. After all,
spanning tree's only job is to prevent loops by shutting off redundant
paths in the network. But your workstation only has one path to
the network, through that port!

Irfan

So we can turn off spanning tree?

Gillian

Well, were not going to turn off spanning tree, we'll just disable it
on this one port. But now that you mention it, it very possible
that you are the administrator of a network that by physical design,
does not have any loops in the topology. Nearly all small networks
are like that. If that's the case, yes, you can disable spanning
tree network wide and save the bandwidth and CPU cycles that STP would
use to do a job you don't need.

Irfan

My network has redundant paths, so I'll use spanning tree, but can
you tell me exactly how to shut it off on a single port, so my
workstation can access the network immediately?

Gillian

Certainly, Cisco calls it PortFast
and you enter the following command:

set spantree portfast <module>/<port>enable

So if you plug your workstation into the 12th port on the 2nd slot in
your Catalyst 5505, you would enter the command:

set spantree portfast 2/12 enable

To disable PortFast on this port:

set spantree portfast 2/12 disable

As you can see, those engineers at Cisco made this easy.

Irfan

You call that easy? Cisco should provide a mouse and a
graphical interface like Microsoft does, now that's what I'd call easy...